The [orbit] of a [satellite] about the [Earth] (or anything else, for that matter) is in the shape of a [conic section], with the center of the Earth at one [focus] of the [conic] section.¹ The orbit, therefore, lies in a single [plane] in [space], which also contains the center point of the Earth.

The Earth's [equator] also lies in a single plane in space, containing the center point of the Earth.

These two planes, the orbital and and equatorial, [intersect] in a [line] passing through the center of the Earth. This line is called the "[node]" of the orbit.

The node line is often depicted as two [vector]s, separated by 180 degrees, pointing away from the Earth's center along the node. The [ascending node] is the vector intersected by the orbit as the satellite moves from [south] to [north]. The [descending node] is the opposite: the vector intersected by the orbit as the satellite moves from north to south.

One of the six [classical] [orbital elements]² is the [right ascension of ascending node|right ascension of the ascending node] (Ω). Ω describes the [angle] in the [equatorial plane] between the ascending node and the vector Î, which points from the Earth's center to the [vernal equinox]. In older books, Ω is sometimes called the [longitude of the ascending node] - this is incorrect; it is not a [longitude]. Ω is sometimes, in [shorthand], referred to simply as the "node"; it should usually be clear from [context] whether "node" refers to an angle (Ω) or a line (the intersection of the two planes above).

¹Orbits come in [ellipse|elliptical], [circle|circular], [parabola|parabolic], and [hyperbola|hyperbolic] shapes. Since these are formed by the intersection of a [cone] and a plane, they each lie in a single plane.

²Six elements are required to completely describe the [state] of an orbit. There are many different sets of these elements. The "classical" (or [Kepler]ian) elements are among the most [intuitive] of these, and consist of the [semimajor axis] (a), the [eccentricity] (e), the [true anomoly] (ν) (that's a [nu]), the [inclination] (i), the [right ascension] of the ascending node (Ω), and the [argument of perigee] (ω).

In [finite element modeling], a body being acted upon by a [force] is divided up into many [segment]s ([element]s). In each [cross-section] thus revealed (that is, on each end of each segment), a single [point] is chosen for the purpose of [analysis]. This point is called a [node].

Node (thing)